Thousands were feared dead in New Orleans yesterday as officials said the flood-ravaged city would be abandoned to the waters after the last desperate survivors were evacuated.

Eighty percent of the city remained under water, and authorities all but surrendered the streets to looting and lawlessness. Gunfire crackled on and off as looters roamed the streets in hundreds.

Mayor Ray Nagin warned that the death toll could reach into the thousands.

“We know there is a significant number of dead bodies in the water,” and other people dead in attics, he said. Asked how many, he replied, “Minimum, hundreds. Most likely, thousands.”

That would make Katrina the nation’s deadliest hurricane since 1900, when a storm in Galveston, Texas, killed 6,000 to 12,000 people.

Nagin’s chilling words came as officials drew up plans to evacuate some 100,000 people left in the city and abandon the Big Easy for up to four months.

There will be a “total evacuation of the city. We have to,” the mayor said. “The city will not be functional for three to four months. We are looking at 12 to 16 weeks before people can come in.”

Eighty percent of New Orleans’ half a million people obeyed warnings to leave before Katrina struck on Monday, causing damage estimated at $25 billion.

But about 100,000 remained – including 25,000 left homeless by the storm.

They were lodged in the Superdome and a dozen other smaller shelters. But the giant arena was hot and stuffy, with broken toilets and nowhere to bathe.

“It can no longer operate as a shelter of last resort,” the mayor said.

Officials plan to move the refugees to the Houston Astrodome in a huge, two-day convoy of 500 buses. The buses began arriving last night.

Louisiana Gov. Kathleen Blanco said there was no choice but to clear out.

“The logistical problems are impossible and we have to evacuate people in shelters,” she said. “It’s becoming untenable. There’s no power. It’s getting more difficult to get food and water supplies in, just basic essentials.”

Returning early to Washington from a vacation at his Texas ranch, President Bush said the recovery “will take years.”

But he insisted, “I’m confident that with time you’ll get your life back in order, new communities will flourish, the great city of New Orleans will get back on its feet and America will be a stronger place for it.”

On the trip to Washington, Air Force One dipped low enough for the president to view the destruction.

Katrina lashed Louisiana, Alabama, Mississippi and Florida with 140 mph winds Monday, becoming the costliest natural disaster in U.S. history.

On Tuesday, a day after the Big Easy thought it had escaped the storm’s full fury, two levees broke – allowing Lake Pontchartrain to inundate 80 percent of the city, some of which lies 20 feet below sea level.

Around noon yesterday, the Army Corps of Engineers said the water levels between the city and the lake had equalized and the water stopped rising in the city.

The Corps said it planned to use heavy-duty Chinook helicopters to drop five-ton sandbags into the gaps in the levees.

But the agency said it was having trouble getting the sandbags and dozens of 15-foot highway barriers to the sites because the city’s waterways were blocked by loose barges, boats and large debris.

The Pentagon, meanwhile, began mounting one of the largest rescue operations in U.S. history.

It sent four Navy ships to the Gulf Coast with drinking water and other emergency supplies, along with the hospital ship USNS Comfort, search helicopters and elite SEAL water-rescue teams.

The Pentagon also ordered 10,000 more National Guard troops to Louisiana and Mississippi for storm relief, bringing to 21,000 the total deployed in the four hurricane-battered states.

American Red Cross workers from across the country converged on the devastated region in the agency’s biggest-ever relief operation.